In a wind turbine generator that supplies generated electric power to the utility grid, even if the voltage at an interconnection point between the wind turbine generator and the utility grid suddenly drops due to a momentary power interruption caused by a fault occurring on the utility grid, the wind turbine generator should continue to operate without being disconnected from the utility grid (LVRT: Low-Voltage Ride-Through).
FIG. 7 is a graph showing an example of a voltage drop at a interconnection point. In the example in FIG. 7, the voltage becomes 0 for 150 ms, and then the voltage recovers to 0.9 PU three seconds after the voltage drop; with LVRT however, the wind turbine generator should continue to operate even during the period until the voltage recovers.
However, when a momentary power interruption or the like occurs in wind turbine generators, causing the generator output to drop suddenly, the output power becomes small relative to the input power to a synchronous generator, and the rotor of the generator is accelerated by surplus energy during the period until the generator input power is reduced.
In a system in which an induction generator is directly coupled to the utility grid, or a system in which the generator is coupled to the grid via an inverter, over-rotation of the generator due to the surplus energy should be prevented.
On the other hand, in a system in which a synchronous generator is directly coupled to the utility grid, the input energy to the generator should be instantaneously suppressed with high precision, or an increase in the rotational speed should be suppressed by consuming the output energy of the generator to control the generator so that it does not fall out of synchronization.
Therefore, Patent Literature 1, for example, describes a control method in which a damping resistance close to the target generator is turned on/off to cause the surplus energy to be consumed by the damping resistance in the form of electric power, in order to suppress acceleration of the generator in the system when a fault occurs on the utility grid, thus stabilizing the system.